This invention relates to a vacuum brake power booster for a slip-controlled hydraulic brake unit, with at least two power chambers separated from each other by a first movable wall within a booster housing. One of the chambers being connectable either to a vacuum source or to the atmosphere and the other one, for the purpose of generating a boosting power which is proportional to an input power introduced at a brake pedal, being aeratable through a control valve which is actuatable by means of the brake pedal. The control valve housing comprises two control housing parts being slidable relative to each other. The first control housing part is abutted against a stationary stop and the second control housing part bearing the first movable wall, guiding a control valve piston which is connected to the brake pedal by means of an input member and interacting with a power output member. A second movable wall which is subjectable to a controllable differential pressure and which separates a first pneumatic power chamber connected to the vacuum source in the release position from a second pneumatic power chamber subjectable to the atmosphere and, respectively, to vacuum.
A vacuum brake power booster of this type is known from published German patent application No. P 38 14 222.8. The special feature of the known brake power booster is that in the event of slip control, the second movable wall is movable in a direction contrasting that of actuation of the master brake cylinder due to the effect of two compensating cylinders being associated to the pressure chambers of the master brake cylinder and in that a hydraulic chamber being in active connection with the power output member is provided whose link to a pressureless hydraulic fluid reservoir is cut off by means of a seat valve.
In the known arrangement, the sophisticated set-up, in particular the brake pressure modulator of the brake power booster being connected with considerable design effort, is disadvantageous. In addition, the risk that upon an extended pause of functioning, the components being operated in the event of the anti-locking or the traction slip control action can be damaged, for example, be fretting corrosion and may so become unserviceable also is a disadvantage.
It is, therefore, an object of the present invention to provide a brake power booster of the type mentioned in such a manner that both reliability in operation is increased and, at the same time, the costs of manufacture and of mounting involved are reduced to a considerable extent. Moreover, the master cylinder pistons and the brake pedal are to be completely restored upon each slip control action.